Lesson #1 - Top Down Design

Last updated: Mon Feb 18 09:27:28 PST 2002

Programs are best thought of as a list of instructions, which can be seen group together by purpose.

At the very top level, you might have:

BEGIN
DO 'program';
END.

This is just a reference to another set of instructions. BEGIN & END note where your program runs from and too, marking off a block of instructions that you could denote at a higher level as one item.

From here, we could expand on the 'program' instruction:

DEFINE 'program'
BEGIN
        DO 'get data input by user';
        DO 'manipulate data';
        DO 'display data as output';
END;

BEGIN
DO 'program';
END.

The three stages of the 'program' module listed here are also the key stages of any program. They will all occur in a real program at least once, in one way or another (although you might not realize it sometimes). You could write a program that leaves out some of the steps, but it would be mostly useless.

For purposes of simplification, you could reduce the program to this:

DEFINE 'get data input by user'
BEGIN
END;

DEFINE 'manipulate data'
BEGIN
END;

DEFINE 'display data as output'
BEGIN
END;

BEGIN
        DO 'get data input by user';
        DO 'manipulate data';
        DO 'display data as output';
END.

In this now, our actual instructions are just empty blocks, they do nothing. We could thusly simplify them further. Notice the semi-colon at the end of the line, that tells the system this is the end of an instruction definition.

DEFINE 'get data input by user';
DEFINE 'manipulate data';
DEFINE 'display data as output';

These are called prototypes in many programming languages, because they just tell the system that a certain instruction exists, without giving details about the function.

To answer a question a received about my examples here, what I call instructions are known by several other names in various programming languages. These include sub-routines (Basic), procedures (Pascal) and functions (C/C++ & Pascal).